1. Field of the Invention
This invention relates generally to mechanical and chemical processing of semiconductor wafers, and more particularly concerns a method and apparatus for recovery of components of an aqueous chemical and mechanical abrasive slurry containing finely divided, suspended particles following their use in processing of semiconductor wafers.
2. Description of Related Art
Semiconductor components are commonly manufactured by layering electrically conductive and dielectric materials to achieve appropriate electrical characteristics for fabrication of multiple electrical components such as resistors, capacitors and transistors. Many of these discrete devices are incorporated into integrated circuits for use in creating microprocessors, memory chips, logic circuits, and the like. Many integrated circuits can be produced on semiconductor wafers by layering of dielectric and electrically conductive materials to create multiple semiconductor devices in a relatively small area.
The density of electrical components on such semiconductor devices has continually increased as trace line widths and element sizes on such semiconductor devices have narrowed. At one time, for example, trace line widths on such devices typically ranged from 1 .mu.m to 4 .mu.m. However, in recent years, the industry has made significant advances in reducing trace line widths used in integrated circuits to less than 1 .mu.m. Currently, trace line widths of 0.5 to 0.35 .mu.m are common, and research is being conducted to achieve trace line widths of from 0.25 .mu.m to 0.18 .mu.m. In addition, the demand for increased memory and computing power has driven limits on the number of semiconductor devices per integrated circuit that are achievable ever higher, resulting in an increase in the number of layers applied to semiconductor wafers, while the typical size of the integrated circuits continues to decrease. The combination of narrower trace line widths, increased numbers of layers of materials and higher densities of semiconductor devices per integrated circuit has made such devices increasingly susceptible to failure due to inconsistencies on semiconductor wafer surfaces, and it has become increasingly important that such semiconductor wafers have surfaces and dielectric layers that are uniformly smooth.
Methods for chemical mechanical planarization (CMP) have been developed to polish the surface of semiconductor wafers, and typically involve rotating the wafer on a polishing pad, applying pressure through a rotating chuck, and supplying an aqueous chemical slurry containing an abrasive polishing agent to the polishing pad for both surfactant and abrasive action. The chemical slurry can additionally contain chemicals that etch various surfaces of the wafer during processing. Abrasive agents that can be used in the chemical mechanical slurry include particles of fumed silica, cesium and alumina. The chemical mechanical slurry can also include stabilizer or oxidizer agents. Fumed silica is typically mixed with a stabilizer such as potassium hydroxide or ammonium hydroxide, and is commonly used to polish dielectric or oxide layers on the semiconductor wafer. Cesium and alumina are commonly mixed with an oxidizer agent such as ferric nitrate or hydrogen peroxide, and are typically used to polish metal layers, such as tungsten, copper and aluminum, for example.
The slurry and material removed from the various layers of the semiconductor wafer form a waste stream that is commonly disposed of as industrial waste. The abrasive components constitute approximately 8% to 15% of the raw waste stream, with the remainder constituting other chemical agents such as stabilizer or oxidizer agents, and water. The raw waste stream is typically diluted with rinse water to yield a final solids concentration of approximately 1% to 1.5% in the waste stream. However, the disposal of dissolved or suspended solids in the industrial waste stream has become a relevant issue due to strict local, state and federal regulations, and it would thus be desirable to provide a process and apparatus to remove abrasive components from the waste stream for possible reprocessing for use again in the chemical mechanical slurry or for other reuse or disposal, or for possible removal of heavy metal components for separate disposal. It would also be desirable to treat and reclaim the waste stream supernatant liquid to permit reuse of the supernatant liquid from the chemical mechanical planarization process.
While conventional techniques of reverse osmosis filtration, centrifugation and electrophoreses have been used for reclamation of water from fluids containing large particles typically greater than about 3-4 microns in diameter, such as coal slurry, mud, and effluent from metal plating processes, these techniques do not effectively deal with submicron colloidal suspensions produced in the industrial waste from the semiconductor manufacturing and fine grinding industries. Such techniques are also commonly limited to batch processing or have low throughput volumes, and are not readily adapted to high volume, continuous service. Filters are also typically inefficient and costly for removing fine suspended solids in such waste streams.
Other waste water treatment processes involve pH neutralization and addition of flocculating or settling agents, such as alkaline earth metal oxides, alkaline earth metal hydroxides and cationic polymers, for removal of suspended solids, and decanting of supernatant liquid from settling tanks. However, the use of such flocculating agents can pose serious problems for the reuse of reclaimed solids and reclaimed water for the semiconductor manufacturing industry. Such systems also tend to be large and relatively ineffective in removing fine or colloidal particles, and are also not readily adapted to high volume, continuous service. Because of the large volume of waste generated by the semiconductor manufacturing process, it is desirable to provide the capability for processing the waste stream from the semiconductor manufacturing industry on a continuous, rather than batch, basis.
Conventional techniques for motivating slurries utilize pumps, which are generally suitable for most slurries which are intended to be colloidal in nature. Such slurries are commonly maintained in suspension with minor agitation. The use of pumps such as centrifugal pumps typically cause flocs to break apart, resulting in an increase in the level of finely suspended particles, thereby hindering removal of flocs from the slurries. Diaphragm and peristaltic pumps typically create smaller particles, and therefore further hinder the removal of solids from slurries. It would therefore also be desirable to provide a method and system for separation and recovery of relatively small particles from slurries that motivates the slurry for processing with a minor amount of agitation. The present invention meets all of these needs.